Rare earth-transition metal (RE-TM) magnets have excellent magnetic properties. They are approximately ten times stronger than iron magnets, and five times stronger than ferrite magnets. Their residual magnetic strengths make them particularly suitable for use as pole pieces in small size, high torque, DC motors. RE-TM magnets having long, thin, curved shapes are particularly useful as DC motor pole pieces when they are metallurgically bonded to the inside walls of the motor casing. When the motor casing is made of a ferromagnetic material, such as steel, it not only provides support for the magnets and other motor components, but also serves as a return flux path for the magnets.
By rare earth-transition metal compound is meant, the chemical combination of a transition metal such as cobalt, nickel, iron, manganese and chrome, and a rare earth element such as yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or samarium. The rare earth constituent may also be in the form of mischmetal, naturally occurring or refined combinations of rare earth elements. A preferred magnet composition is, e.g., SmCo.sub.5. The rare earth-transition metals suitable for use herein are those having high energy products. The energy product is a measure of the energy that a magnet material can supply to an external magnetic circuit such as a flux field for a D.C. motor, without demagnetizing.
The physical characteristics of RE-TM powders make them difficult to form into densified curved shapes by standard powder metallurgy methods. The powder particles are extremely hard (approximately Rockwell C-53 for SmCo.sub.5), and have low compressibility. Efficient use of RE-TM materials requires good magnetic alignment. In our earlier work, we discovered a method for pressing RE-TM powders into relatively high density, self-supporting, thin, curved compacts wherein the individual particles were radially magnetically aligned. The compaction required two specially adapted powder metal presses, the first to magnetically align the powder and the second to achieve the high pack density necessary for self-supporting compacts. Moreover, hardened die liners were required to cut down die wear and to transport the powders between presses. The method of making these dense RE-TM powder compacts is described in U.S. Pat. No. 4,123,247. We also discovered a method of using hot isostatic compaction to densify thin, curved RE-TM powder compacts into magnetic pole pieces for cylindrically shaped DC motors. That invention is described in U.S. Pat. No. 4,104,787. A method of densifying curved green powder compacts into permanent magnets by hot isostatic pressing is disclosed in U.S. Pat. No. 4,151,060, and by restrained sintering in U.S. Pat. No. 4,144,060.
Before this invention, it was not known how to form relatively long, thin, curved, rare earth-transition metal magnets, with radially aligned magnetic domains, without first forming self-supporting green powder compacts. Furthermore, the thinness of RE-TM magnets was practically limited by the green strengths of the pressed powder compacts. We have discovered a method of making long, curved, RE-TM magnets as thin as desired.